Root gives you super user access aka SU. It will allow you to flash custom ROMs and themes. Some apps require root to work properly.
Some perks are better battery life, free tethering and removal of fps cap.
tethering is using your phone as a hotspot basically, with rooting you can do that free. For an incredible, which I'm guessing thats what you have, google "unrevoked", its a 1-click program that roots your phone
I would bet that only 1 in 50 Incredible owners have root if that....and most of those nonrooters are probably very content.
Like someone mentioned earlier google "how to root" and "unrevoked" to learn more about it. Rooting your phone gives you as much control over the innards of your phone as you're willing to tinker with...and because of that the results can be bad. However, if you're careful and do your homework you can turn your Incredible into an "effin' Incredible."